In recent years, techniques to form insulated gate transistors using a semiconductor thin film (with a thickness of approximately several nanometers to several hundreds of nanometers) which is formed over a substrate having an insulating surface have attracted attention. The insulated gate transistor is widely applied to electronic devices such as an IC and an electro-optical device and has been expected to be rapidly developed particularly as a switching element for an image display device. Various metal oxides exist and are used for a variety of applications. Indium oxide is a well-known material and is used as a transparent electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides have semiconductor characteristics. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Insulated gate transistors in each of which such metal oxide having semiconductor characteristics is used for a channel formation region have already been known (Patent Documents 1 and 2).